You can find this information here - How Tent Storage Works in DayZ - Scare Bears Clan.
This answer is current as of version 188.8.131.52
Ok so rule 1: In a tent every item counts as 1 slot. That means your m4a1 is 1 slot, a tire is 1 slot, a jerrycan is 1 slot, anything
and everything is 1 slot when inside a tent.
Storage is broken down into 3 different categories, each one has it’s
own amount of storage space.
So an example of a full tent would be:
5 AKM’s, 2 Binoculars, 2 M1911
Pistols, 1 NV goggles. = 10 Slots
5 Alice Packs. = 5 Slots
10 tires, 5
jerrycans, 20 bloodbags, 5 Stanag Ammo, 10 Raw Meat. = 50 Slots.
So as you can see it doesn’t matter how many slots the item would take in
your inventory, it only counts as 1 slot within the tent.
If you attempt to place items into a full tent one of two things can
happen. The item can drop on the floor near the tent or the item could
disappear. So take care when placing items in the tent.
My personal theory on items disappearing after placing in a full tent
is — the items are placed under the tent or perhaps under the ground
if your tent is on a hill.